The invention relates to a transmission system for pulse signals of fixed clock frequency comprising a transmitter, a receiver and a plurality of repeaters located in a transmission path. The receiver and the repeaters are each provided with an input circuit and an equalizing amplifier which is connected thereto and comprises a fixed section for equalizing the nominal transfer characteristic of the preceding transmission path and an adjustable section with associated adjusting circuit for automatically equalizing deviations from the said nominal transfer characteristic. The equalizing amplifier is connected to a pulse regenerator and to a clock extraction circuit for obtaining the clock frequency for controlling the pulse regenerator, an output circuit being connected to the pulse regenerator. Such transmission systems are generally known and are used for the transmission of pulse code modulation signals, synchronous telegraphy and data signals and the like.
In such a transmission system the transfer characteristic of the transmission path, which in many cases is a cable, is a function of the distance between two successive repeaters and of the ambient temperature. To obtain maximum uniformity and simplicity of the design of the repeaters, the equalization required for pulse regeneration is effected in the equalizing amplifier in a fixed section which equalizes the transfer characteristic of a transmission path of nominal length at a nominal temperature and in an adjustable section for automatically equalizing the variations with respect to this nominal transfer characteristic which variations are caused by deviations from the nominal length and from the nominal temperature which are inevitable in practice.
In many applications the adjusting signal for automatic equalization is obtained by means of an adjusting circuit including a peak detector which is connected to the output of the equalizing amplifier. The output signal of the peak detector is used to adjust the equalizing amplifier so that the pulse signals is its output have a constant peak value. However, it has been found that obtaining a reliable adjusting signal according to this method results in that the requirements to be satisfied by the amplifier proper, which particularly in transmission systems for pulse signals of clock frequencies higher than 30 MHz are exacting in any case, become even more stringent and nevertheless correct adjustment of the equalization is not obtained under all conditions.
It is an object of the present invention to provide a transmission system of the type set forth in the preamble in which the amplifier proper does not have to satisfy additional requirements to obtain a reliable adjusting signal for equalization and yet under all conditions correct adjustment of this equalization is ensured, which transmission system is of simple structure and highly suited for pulse signals of very high clock frequencies.